The disclosure relates to a drive unit of a hybrid vehicle. The hybrid vehicle has both an engine (an internal combustion engine) and an electric machine mounted therein. More particularly, the disclosure relates to a drive unit which changes drive modes using two electric machines and two differential gear units.
One example of a drive unit which is connected to an engine and mounted in a hybrid vehicle is a two motor type split hybrid drive unit (see US Patent Application Laid-Open No. 2002-0142876). The hybrid drive unit uses two electric machines (simply referred to in this specification as “motors” or “motor-generators” for the sake of convenience) and two differential gear units, and switches between three and four elements, depending on the running state of the vehicle, by controlling the differential gear units by applying/releasing two of the engagement elements. As a result, the drive unit achieves a three-element reduction mode when the vehicle is run at low speeds, and achieves a four-element direct drive mode when the vehicle is run at high speeds.
With the drive unit in the foregoing publication, however, in the three-element reduction mode (3 Lo), the relationship in which the output Pmg1 of one of the motors that provides motor power cancels out the output Pmg2 of the other motor that receives the reaction force of the output Pmg1, increases and decreases linearly depending on the vehicle speed, as shown by the operating characteristics in FIG. 12. On the other hand, in the four-element direct drive mode (4 Hi), this relationship changes in a curve shape with respect to the vehicle speed. The rate of increase in the output of the motor providing motor power increases as the vehicle speed increases. As used herein the “four-element direct drive mode” refers to the state that the rotational speed of the output member is the same as the rotational speed of the planetary gear element, which is connected to the output member. As used herein the term “three-element direct drive mode” refers to the mode that includes two states that the rotational speed of the output member is the same as the rotational speed of the planetary gear, which is connected to the output member, and the rotational speed of the second electric machine (MG2) is the same as the output member.